This invention relates to a device for measuring the rate of flow of flowable products, particularly loose material, comprising a balance beam pivoted to a housing, an inclined measuring plate, adapted to deflect the product flow supplied in metered quantities and discharged by gravity, mounted on the balance beam in a position in which its measuring surface extends substantially parallel to the longitudinal axis of the balance beam, and a mechanism provided in the housing and associated with the balance beam arm remote from the measuring plate and which is intended for acting on such balance beam arm in opposition to the effort of the deflecting force acting on the measuring plate, while a variable, proportional to the time rate of the product flow and represented, preferably, by a pneumatic signal resulting from the oscillation of the balance beam about the position of equilibrium thereof, constitutes the basis of the measurement or flow control derived therefrom.
In a first group of known devices of this kind, the action of force produced by the deflection of a product stream flowing over a measuring plate is utilized for determining the respective time rate of flow or controlling the same, by means of a measuring plate extending substantially parallel to the horizontal axis of the pivot. In some of these designs, the measuring plate is provided at one end of a balance beam centrally pivoted to a fixed support, and the measuring surface extends transversely to the longitudinal direction of the beam.
There are also known solutions in which the measuring plate itself is pivoted to a fixed support, oriented substantially parallel to the horizontally extending pivot axis and operationally connected to a device for measuring or controlling the product stream, by means of a transmission lever.
In both cases, the proportionality between the force acting on the device and the resultant variation of the time rate of flow of the product is affected by the fact that, due to the displacement of the point of application, on the measuring plate, of the resulting deflecting force, resulting from a variation of the weight rate of flow, the length of the lever arm for this force does not remain constant.
For eliminating the variations of the lever arm length, it has been proposed to provide a measuring plate with a measuring surface extending in the longitudinal direction of the balance beam carrying the plate. The design of the metering device mounted thereabove insures that, during variations of the weight rate of flow, the product stream which, considered in the longitudinal direction of the balance beam, flows symmetrically of its stationary centroidal axis, is increased or decreased.
Such an arrangement, based on the principle of a lever scale, comprises a balance beam swingable in a vertical plane. The axis of rotation of the balance beam pivot which consequently extends horizontally, permits utilizing only the vertical component of the resulting deflecting force at the measuring plate for the measurement.
In another device of this kind, the balance beam is mounted on a fixed support for oscillation about an axis which extends vertically. In this device, the balance beam can swing only horizontally so that only the horizontal component of the resulting deflecting force can be used for measuring.
However, experience has shown that, for the required accuracy of measurement of the control depending thereon, it is not sufficient merely to maintain the moment arm constant. The varying physical properties of the product, becoming manifest by a varying coefficient of friction, cause errors of measurement even with a constant volume flow.
It has been found that, on a measuring plate of any geometry, the lines of action of the deflecting forces, the resultants of which are of different magnitude and direction, depending on the different coefficients of friction of the products, do not necessarily intersect in a single point. The scatter of the point of application of the deflected forces on the measuring plate, caused by the varying friction factors, results in measuring errors even in cases where the measuring plate itself is mounted on a fixed support for swinging about a horizontally extending hinge axis, with the transmission of the measuring force to the measuring or control members by means of a lever arm varying with the friction coefficient of the product and in a vertical plane comprising the centroidal axis of the products stream and passing through the measuring surface in a direction normal to the hinge axis.
By using a balance beam carrying a measuring plate which is oriented in the longitudinal direction thereof, the transmission of the forces from the plate to the measuring or control members is transferred into a plane extending at a right angle to the plane containing the centroidal axis of the product in the arrangement with the measuring plate hinged directly to the support. This makes it possible to disregard a control, otherwise required by the varying friction factors, of the points of application of the respective deflecting forces on the measuring plate.
The length of the balance beam portion between the centroidal axis of the product stream, fixed by an appropriate design of the metering device, and the axis of rotation of the balance beam pivot remains constant, as a lever arm, for any deflecting force having its point of application situated in the plane which contains the centroidal axis of the product stream and which is normal to the longitudinal axis of the balance beam in its equilibrium position.
Due to such an arrangement, only one component of the respective deflecting force acting on the measuring plate is available for the measuring. As is well known, in the lever scale arrangements of the prior art, while utilizing the vertical or horizontal component of the resulting deflecting force, a deflecting torque is produced in the respective vertical or horizontal plane, depending on which of the two components is used as the measuring force to which the axis of rotation of the balance beam pivot is perpendicular.
In a device for utilizing the vertical component, with a constant flow volume, a deficit increasing with the coefficient of friction invalidates the actual time rate of flow.
In a device based on the measurement of the horizontal component, on the contrary, the effect of the same increase of the coefficient of friction is that, with the same unchanged flow volume, the actual time rate of flow is measured as increasingly diminishing with the increasing deficit.
Since, as a rule, the magnitude of the horizontal component is smaller than that of the vertical component, the relative influence of the friction is more important in the latter device.